A microwave heater includes a magnetron that constitutes a microwave generator. The magnetron has a main body including an antenna part, an anode part, and a cathode part.
A general configuration of a conventional magnetron will be described below with reference to FIGS. 9 and 10.
FIG. 9 is a vertical cross-section view of a configuration of a main body of the conventional magnetron. FIG. 10 is a vertical cross-section view of antenna part 101 in the conventional magnetron.
As illustrated in FIGS. 9 and 10, the magnetron includes antenna part 101, anode part 102, cathode part 103, and the like. Antenna part 101 holds internally antenna conductor 104 guiding a microwave. Anode part 102 holds internally a plurality of vanes 105 radially arranged. Cathode part 103 holds internally cathode lead wire 107.
Vanes 105 in anode part 102 are connected to antenna conductor 104 derived to antenna part 101. Filaments 106 as cathodes are arranged in the center of vanes 105 in anode part 102. Filaments 106 are connected to cathode lead wire 107. Cathode lead wire 107 is connected to input terminal 109 via terminal-side ceramic stem 108.
Antenna part 101 of the magnetron has antenna conductor 104 derived from anode part 102 and connected to exhaust pipe 112 via output-side ceramic stem 111. Exhaust pipe 112 has chip-off portion 113. With chip-off portion 113, the main body of the magnetron in which anode part 102, cathode part 103, and antenna conductor 104 are surrounded by vacuum walls can be maintained under vacuum.
Chip-off portion 113 has a sharpened portion. Accordingly, antenna cap 110 is press-fitted to protect chip-off portion 113 of exhaust pipe 112. Antenna cap 110 is arranged in a wave guide to serve as part of an output terminal. Accordingly, antenna cap 110 radiates a microwave generated in the main body of the magnetron to the wave guide.
The general magnetron is configured as described above.
In the thus configured magnetron, antenna cap 110 may fall off exhaust pipe 112 as described below.
Specifically, the magnetron is repeatedly used for a long period of time with large temperature differences between an operating time for generating a microwave and a non-operating time. This weakens a degree of pressure bonding between press-fitting portions of antenna cap 110 and exhaust pipe 112. Accordingly, antenna cap 110 may fall off exhaust pipe 112.
When antenna cap 110 falls off exhaust pipe 112, electric discharge occurs between exposed exhaust pipe 112 and fallen antenna cap 110 or the wave guide. Accordingly, the magnetron becomes unstable in operation. Further, in some cases, exhaust pipe 112 may have a vacuum leak, for example.
Thus, to prevent antenna cap 110 from falling off exhaust pipe 112, there have been conventionally proposed magnetrons of various configurations (for example, refer to PTL 1, 2, and 3).
The magnetron described in PTL 1 has a projection on the inner peripheral surface of an antenna cap and a concave groove formed in the outer peripheral surface of an exhaust pipe onto which the antenna cap is press-fitted. The projection in the antenna and the concave groove in the exhaust pipe are fitted together to prevent the falling of the antenna cap.
In the magnetron described in PTL 2, an antenna cap is formed of a composite plate in which a foundation of an iron-based material is coated with a surface skin film of an alloy of silver and copper and an anti-oxidation film made of the same material as that for the surface skin film. Then, the composite plate is subjected to a drawing process to form the antenna cap. Accordingly, the antenna cap is enhanced in mechanical strength to prevent the falling off the exhaust pipe.
In the magnetron described in PTL 3, a hole is formed in the side surface of the antenna cap, the antenna cap is press-fitted onto the exhaust pipe, and the hole in the antenna cap and the exhaust pipe are spot-welded. This configuration prevents the falling of the antenna cap.
As described above, the conventional magnetrons are configured such that the antenna cap and the exhaust pipe are basically press-fitted and firmly fixed to prevent the antenna cap from falling off the exhaust pipe.
However, an experiment result described below has revealed that the configuration of the magnetron described in PTL 1 cannot prevent the occurrence of electric discharge in a gap between the antenna cap and the exhaust pipe.
Specifically, the inventors of the prevent application have experimentally observed electric discharge between the antenna cap and the exhaust pipe in the magnetron configured as described above. In particular, the inventors have found that, when there is a gap between the edge of the press-fitting portion in the antenna cap and the portion of the exhaust pipe joined to the output-side ceramic stem, electric charge would occur between the edge and the joined portion. Specifically, the inventors of the present application have found that electric discharge would occur when the gap between the edge and the joined portion has a spatial distance of 0.2 mm as a design dimension. The inventors of the present application attribute the electric discharge to the slightly widened and flared edge of the press-fit portion in the antenna cap. That is, a potential difference is generated by a creepage distance between the edge of the press-fit portion and the joined portion of the exhaust pipe and the output-side ceramic stem. This potential difference is considered to cause the electric discharge.
In a general magnetron, both the antenna cap and the exhaust pipe firmly fixed to each other by press-fitting are formed in the shape of a cylinder. In this case, it is not possible in the terms of mechanical processing to form the two cylinders in a true circle and bring them into absolute contact with each other along the entire circumference. Specifically, the conventional antenna cap is formed by subjecting a thin-plate metallic material of stainless, brass, aluminum, or the like to a drawing process. Accordingly, the press-fit portion of the antenna cap is formed in an almost triangular or almost square open shape with corners to be in point-contact with the cylindrical exhaust pipe at at least three points. This creates a gap between the edge and the joined portion to cause electric discharge.
In addition, the magnetrons described in PTL 2 and PTL 3 are configured such that the antenna cap is press-fitted and firmly fixed to the exhaust pipe as in the foregoing case. Accordingly, the edge of the antenna cap is formed in a slightly flared shape. The press-fit portion is press-fitted by partial contact without close adhesion along the entire circumference. This creates some gap between the antenna cap and the exhaust pipe. As a result, electric discharge may occur in the formed gap.
A microwave heater used in a household microwave oven or the like generally uses a microwave of 2.45 GHz. In addition, an industrial microwave heater uses a high-frequency microwave of 5.0 GHz or more, for example, 5.8 GHz. That is, the magnetron generating a higher-frequency microwave has a shorter wavelength. Accordingly, when the antenna cap is press-fitted into the exhaust pipe, electric discharge is more likely to occur even in a slight gap. Therefore, there is demand for a magnetron that can suppress electric discharge in a more reliable manner.